Dougal: Almost all computing tasks suffer a bottleneck somewhere, whether it be memory access, disk or CPU. The Risc PC's slow IDE disk interface has been "described as a bottleneck" too; so has the lack of hardware floating point.

The point you're missing is that most RISC OS users are more interested in the performance of the applications they're actually running, rather than being told from on high that "XYZ is a bottleneck". You are correct in that most modern operating systems struggle with memory access to the processor (hence ever increasing L2 cache sizes, amongst other things), but RISC OS and its applications are far less affected by this.